under-constrained symbolic execution: correctness checking ... · under-constrained symbolic...

U N D E R - C O N S T R A I N E D S Y M B O L I C E X E C U T I O N : C O R R E C T N E S S C H E C K I N G F O R R E A L C O D E

2 4 T H U S E N I X S E C U R I T Y S Y M P O S I U M A U G U S T 1 2 , 2 0 1 5

D A V I D A . R A M O S A N D D A W S O N E N G L E R S TA N F O R D U N I V E R S I T Y

David A. Ramos & Dawson Engler USENIX Security 2015

C O N T R I B U T I O N S

• Technique + tool for finding deep bugs in real, open source C/C++ code

‣ No manual testcases

‣ No functional specification

• Bugs reported may have security implications; exploitability must be determined manually

‣ Memory access, heap management, assertion failures, division-by-zero

• Found 77 new bugs in BIND, OpenSSL, Linux kernel

‣ 2 OpenSSL DoS vulnerabilities: CVE-2014-0198, CVE-2015-0292

‣ 14 Linux kernel vulnerabilities (mostly minor DoS issues)

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M O T I VAT I O N : C U R R E N T P R A C T I C E

• Code reviews

• “Safer” languages

• Manual (regression) testing

• Static analysis (Coverity, clang static analyzer, etc.)

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Bugs are everywhere!


S Y M B O L I C E X E C U T I O N

• Provide symbolic rather than concrete inputs

• Conceptually: explore all paths through a program

• Accurately track all memory values (bit precision)

• Report paths/inputs that crash

‣ Generate concrete testcase

• KLEE tool (prior work: OSDI 2008)

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int foo(int x) { if (x) return x/10; else return 10/x;}

x is symbolic input

E X A M P L E


int foo(int x) { if (x) return x/10; else return 10/x;}

symbolic branch

E X A M P L E


E X A M P L E

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if (x) return x/10;

else return 10/x;

x != 0 x == 0

State 1 State 2

Division: OK


E X A M P L E

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Division: ERROR


P R O B L E M : S C A L A B I L I T Y

• Path explosion

‣ | paths | ~ 2 | if-statements |

• Path length and complexity

‣ Undecidable: infinite-length paths (halting problem)

• SMT query complexity (NP-complete)

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S O L U T I O N : U N D E R - C O N S T R A I N E D

• Directly execute individual functions within a program

‣ Less code = Fewer paths

‣ Function calls executed (inter-procedural)

‣ Able to test previously-unreachable code

• Challenges

‣ Complex inputs (e.g., pointer-rich data structures)

‣ Under-constrained: inputs have unknown preconditions

- False positives

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U C - K L E E T O O L

• Extends KLEE tool (OSDI 2008)

• Runs LLVM bitcode compiled from C/C++ source

• Automatically synthesizes complex inputs

‣ Based on lazy initialization (Java PathFinder)

‣ Supports pointer manipulation and casting in C/C++ (no type safety)

‣ User-specified input depth (k-bound) [Deng 2006]

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L A Z Y I N I T I A L I Z AT I O N

• Symbolic (input) pointers initially unbound

• On first dereference:

‣ New object allocated

‣ Symbolic pointer bound to new object’s address

‣ Assume no aliasing (i.e., no cyclical data structures)

• On subsequent dereferences:

‣ Pointer resolves to object allocated above

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int listSum(node *n) { int sum = 0; while (n) { sum += n->val; n = n->next; } return sum;}

E X A M P L E unbound symbolic input



E X A M P L E



n NULLn == 0E X A M P L E

n

n != 0



n NULLn == 0E X A M P L E

n

n != 0

uc_node1

next

val

n == &uc_node1



n NULL

n

n == 0

n != 0

uc_node1

next

val

n == &uc_node1

E X A M P L E



n NULL

n

n == 0

n != 0

uc_node1

next

val

n == &uc_node1

n

n != 0uc_node1

next

val

n == &uc_node1

NULL

uc_node1.next == 0

E X A M P L E



n NULL

n

n == 0

n != 0

uc_node1

next

val

n == &uc_node1

n

n != 0uc_node1

next

val

n == &uc_node1

NULL

uc_node2

next

val

uc_node1.next == &uc_node2

uc_node1.next == 0

E X A M P L E



n NULL

n

n == 0

n != 0

uc_node1

next

val

n == &uc_node1

n

n != 0uc_node1

next

val

n == &uc_node1

NULL

uc_node2

next

val

NULL

uc_node1.next == &uc_node2 uc_node2.next == 0

uc_node1.next == 0

E X A M P L E


U S E C A S E S

• Equivalence checking: patches

‣ Yesterday’s code vs. today’s code (i.e., fewer bugs today)

‣ Goal: detect (and prevent!) new crashes introduced by patches

‣ Other uses discussed in CAV 2011 paper

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PAT C H E S

Source: https://twitter.com/phabricator

https://twitter.com/phabricator


U S E C A S E S


‣ Yesterday’s code vs. today’s code (i.e., fewer bugs today)

‣ Goal: detect (and prevent!) new crashes introduced by patches

‣ Other uses discussed in CAV 2011 paper

• General bug-finding: rule-based checkers ‣ Single version of a function; under-constrained + additional checker rules

‣ Memory leaks, uninitialized data, unsafe user input

‣ Simple interface for adding new checkers

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E Q U I VA L E N C E C H E C K I N G

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retA = fooA(x);retB = fooB(x);

assert(retA == retB);

identical input (symbolic)

assert equivalence



• Value equivalence

‣ Return value

‣ Arguments passed by reference

‣ Global/static variables

‣ System call effects (modeled)

• Error (crash) equivalence

‣ Both versions typically have the same same (unknown) preconditions!

‣ Neither version crashes on an input

‣ Both versions crash on an input

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USE CASE: whether patches introduce

crashes



• Check per path equivalence of two functions

• If all paths exhausted, equivalence verified (up to input bound)

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E VA L U AT I O N

• BIND, OpenSSL

‣ Mature, security-critical codebases (~400 KLOC each)

• Patches

‣ BIND: 487 patches to 9.9 stable (14 months)

‣ OpenSSL: 324 patches to 1.0.1 stable (27 months)

• Ran UC-KLEE for 1 hour on each patched function

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E VA L U AT I O N : PAT C H E S

• Discovered 10 new bugs (4 in BIND, 6 in OpenSSL)

‣ 2 OpenSSL DoS vulnerabilities:

- CVE-2014-0198: NULL pointer dereference

- CVE-2015-0292: Out-of-bounds memcpy read

• Verified (w/ caveats) that patches do not introduce crashes

‣ 67 (13.8%) for BIND, 48 (14.8%) for OpenSSL

‣ Caveat: max. input size (25KB), tool limitations/bugs

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O P E N S S L C V E - 2 0 1 4 - 0 1 9 8

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do_ssl3_write():1 if (wb->buf == NULL)2 if (!ssl3_setup_write_buffer(s))3 return -1;4 ...5 /* If we have an alert to send, lets send it */6 if (s->s3->alert_dispatch) {7 i=s->method->ssl_dispatch_alert(s);8 if (i <= 0)9 return(i);10 /* if it went, fall through and send more stuff */11 }12 ...13 unsigned char *p = wb->buf;14 *(p++)=type&0xff;

NULL pointer check

call sets wb->buf to NULL

NULL pointer dereference


O P E N S S L C V E - 2 0 1 4 - 0 1 9 8

• Uncommon code path

‣ SSL_MODE_RELEASE_BUFFERS runtime option (used by Apache mod_ssl)

‣ SSL alert pending (could be triggered by attacker)

‣ Difficult to consider this case with traditional testing

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FA L S E P O S I T I V E S

• Function’s inputs have unknown preconditions

• Partial solutions

‣ Automated heuristics

‣ Manual annotations (lazily, as needed)

- Written in C/C++, separate from codebase

- Simple annotation can silence many errors

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FA L S E P O S I T I V E S : E X A M P L E ( B I N D )

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1 int isc_region_compare(isc_region_t *r1, isc_region_t *r2) {2 unsigned int l;3 int result;4 5 REQUIRE(r1 != NULL);6 REQUIRE(r2 != NULL);78 l = (r1->length < r2->length) ? r1->length : r2->length;910 if ((result = memcmp(r1->base, r2->base, l)) != 0)11 return ((result < 0) ? -1 : 1);12 else13 return ((r1->length == r2->length) ? 0 :14 (r1->length < r2->length) ? -1 : 1);15 }

INVARIANT(r->length <= OBJECT_SIZE(r->base));

623 errors silenced (7.5% of all errors reported for BIND)


M A N U A L A N N O TAT I O N S

• BIND: 400 lines of annotation code (~0.1%)

• OpenSSL: 60 lines of annotation code (~0.02%)

• Reasonable effort relative to code size (~400 KLOC) and importance

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G E N E R A L B U G - F I N D I N G

• Run single version of a function (w/ lazy initialization)

• Individual checkers look for specific types of bugs:

‣ Leak checker

‣ Uninitialized data checker

‣ User input checker

• Like Valgrind but applied to all execution paths

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E VA L U AT I O N

• 20,000+ functions: BIND, OpenSSL, Linux kernel (~12 MLOC)

• Found 67 new bugs

‣ 37 memory leaks

- Linux kernel: exploitable AUTH_GSS leak in NFS SunRPC layer

‣ 19 uses of uninitialized data

- BIND: DNS UDP port PRNG selected by uninitialized value

- Linux kernel: leak of private kernel stack data via firewire ioctl

‣ 11 unsafe user input (Linux kernel only)

- VMware VMCI driver: unchecked memcpy length (~Heartbleed)

- CEPH distributed file system: division-by-zero (kernel FPE)

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U S E R I N P U T C H E C K E R

• User input is fully-constrained (an attacker may supply any value); no unknown input preconditions

• Checker tracks whether each symbolic byte is UC/FC

• Checker emits UNSAFE_INPUT flag if error is caused by FC input

• Suppresses flag for inputs possibly sanitized (false pos. trade-off)

• C annotations: specify functions returning user input

‣ Linux: get_user, copy_from_user, syscall args

‣ BIND: isc_buffer_getuint8

‣ OpenSSL: byte-swaps (n2s, n2l, etc.) [Chou]

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K E R N E L V M C I V U L N E R A B I L I T Y

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1 static int dg_dispatch_as_host(...,2 struct vmci_datagram *dg) {3 dg_size = VMCI_DG_SIZE(dg);4 ...5 dg_info = kmalloc(sizeof(*dg_info) +6 (size_t) dg->payload_size, GFP_ATOMIC);7 ...8 memcpy(&dg_info->msg, dg, dg_size);9 ...10 }

copy_from_user()Fully constrained

Unchecked memcpy length

Send up to 69,632 bytes from host private kernel memory to guest OS

Similar to Heartbleed! (much lower impact)


C O N C L U S I O N

• Under-constrained symbolic execution


• General bug-finding: rule-based checkers

• Experimental results: BIND, OpenSSL, Linux kernel

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@ramosbugs

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